The inventor of the present invention has determined that there are numerous shortcomings with the methods and apparatus of the background art relating specifically to electro-hydraulic actuators.
Electro-hydraulic actuators that are required to maintain a high level of accuracy are typically controlled with servovalves and use position feedback to achieve closed loop control, e.g., with electrical devices. The position feedback may be accomplished by electrical devices such as LVDTs (Linear Variable Differential Transformer), RVDTs (Rotary Variable Differential Transformer), potentiometers, resolvers, Hall effect sensors, or piezo-resistive sensors.
However, there are applications where accurate electro-hydraulic actuators are required and electrical feedback is not available. In these situations, mechanical feedback must be used to maintain closed loop control. In a single-stage type, electro-hydraulic servovalve, a mechanical leaf spring or other spring force from the actuator is normally used to provide the mechanical feedback. As the size or the slew velocity of the actuator increases, the volumetric flow demand eventually exceeds the capacity of the servovalve, e.g., a jet-pipe, and a higher capacity flapper orifice design or a two-stage servovalve is typically required.
The present inventor has determined that it is extremely difficult to attempt to set a higher capacity two-stage servovalve with mechanical feedback from both the second stage as well as the actuator piston (first stage). In addition, the inventor of the present invention has determined that the accuracy of electro-hydraulic servovalves with mechanical feedback is typically limited to only eight percent or higher.
There are several examples of electro-hydraulic servovalves relating to the foregoing discussion of the background art. For example, U.S. Pat. No. 4,335,645 to Leonard, the entirety of which is hereby incorporated by reference, describes a direct drive, two-stage electro hydraulic servo valve incorporating hydro-mechanical position feedback.
However, the inventor of the present invention has determined that this type of complex electro-hydraulic servovalve is relatively expensive and difficult to utilize in practice. As aforementioned, attempting to set a higher capacity two-stage servovalve with mechanical feedback such as that described in the Leonard patent from both the first and second stage is extremely difficult. In the fluidic repeater described by Leonard, the second stage of the servo valve is hydraulically controlled by mechanical feedback from the position piston.
U.S. Pat. No. 4,4450,753 to Basrai et al., the entirety of which is herein incorporated by reference, describes an electro-hydraulic proportional actuator. However, this system requires electrical position feedback. Specifically, a pair of three way solenoid valves is used to position an actuator assembly having a double-acting, linear piston. An electronic control circuit using electrical position feedback nulls out the system and operates the solenoid valves to control fluid flow through the respective ports of the solenoid valves.
U.S. Pat. No. 4,807,517 to Daeschner, the entirety of which is hereby incorporated by reference, describes an electro-hydraulic proportional actuator including at least a piston, a valve for driving the piston into operating positions, a solenoid for producing a driving force for controlling the valve, and a plurality of springs for biasing the piston, solenoid and valve components. In this actuator, compression spring(s) are directly applied to the solenoid plunger and a sliding link rod to produce a force that counters the magnetic pull of the solenoid coil. When an electrical control signal is zero, the compression spring will force the solenoid plunger and the spool rod to the right (as seen and shown in FIG. 1 of Daeschner).
However, as aforementioned, it has been determined that the electro-hydraulic servovalves with mechanical feedback of the background art suffer from the above-described limitations, including being limited in their accuracy to eight percent or higher error rates.